In existing mobile ad hoc networks (MANET), MAC protocol designs can be broadly divided into time-division-multiple-access (TDMA)-based resource allocation and contention based random access protocols. The TDMA-based protocol attempts to determine network connectivity first (i.e., discover the neighbor of each node) and then allocate collision-free channels to links between radio neighbors. The task of allocation of channels (i.e. TDMA slots, frequency hopping, or spread spectrum codes) among nodes becomes very expensive when the nodes are densely located. To ease the assignment problem a hierarchical structure is formed in the network to localize groups of nodes and make the assignment more manageable. However, the management of hierarchical tiers can be energy-expensive for the sensor network.
Bluetooth™ devices are close commercial peers to sensor networks. The Bluetooth protocol is designed to support low bandwidth and short distance (less than 10 m) wireless connections. The Bluetooth topology is a star network where a master node can have up to seven slave nodes to form a piconet. Each piconet uses a centrally assigned TDMA and frequency hopping (FH) pattern. The scalability of a TDMA-based protocol is not as good as that achieved with contention-based protocols. The DCF (distributed coordination function) of IEEE 802.11 is an example of contention-based CSMA/CA (carrier sense multiple access/collision avoidance), and is widely used in MANET because of its simplicity and robustness to the hidden terminal problem. However, the DCF consumes energy continuously because it involves periodically listening to the channel while in idle state. Thus, the conventional random channel access schemes are not suitable for sensor networks.
A power save mechanism (PSM) for the IEEE 802.11 DCF is proposed to periodically listen and sleep to reduce energy consumption. The sleep schedules of all nodes in the network must be synchronized together. That is, they listen at the same time and go to sleep at the same time. However, the PSM in 802.11 is designed for a single-hop network. A sensor-MAC protocol is designed to operate in a multi-hop network and does not assume that all nodes are synchronized together. Again, sensor nodes broadcast periodically a synchronization packet to their immediate neighbors to coordinate their schedule, when to sleep and when to listen. Neighboring nodes still need to periodically update each other with their schedule to prevent long-time clock drift even in sleep mode. Channel access and node wakeup are integrated together in the sensor-MAC protocol.
Power Aware Multi-Access Protocol with Signalling (PAMAS) has been proposed to reduce energy consumption using a second radio channel to detect activity on its neighbors and turning on its main radio in response to such activity. PAMAS does not attempt to reduce idle listening. Topology Manangement for Energy Efficient Sensor Networks (STEM) also use a dual frequency channel setup to wakeup a neighbor and to transmit. The initiator wakes up its neighbors through the wakeup channel and waits for the acknowledgment (ACK) before turning on the data channel. A node must still wake up periodically from its sleep state to listen if any node wants to contact it. Adaptive Self-Configuring Sensor Networks Topologies (ASCENT) adaptively elect active nodes based on measured local connectivity and packet loss information. Active nodes stay awake all the time and perform multi-hop routing, while the rest of the nodes remain passive and periodically check to determine if they should become active.
Time synchronization is a crucial MAC component in wireless sensor networks. Conventional time synchronization schemes like NTP (network time protocol) cannot be applied to sensor networks. The multi-hop time synchronization protocol (MTSP) establishes a time synchronization tree rooted at the fastest node in the wireless network. The MTSP makes the network time converge to a steady state and guarantees an upper bound on the synchronization error in the steady state. This synchronization error bound on the offsets and clock drifts are also used to achieve the tight clock precision. However, a timing stamp at the MAC layer instead of the process generated message approach avoids the well known problem of imperfect physical clock synchronization.
A conventional sensor node achieves the reduction of energy consumption by employing the sleep state in an idle mode, as shown in FIG. 2. A sensor node is referred to as “active” when it is transmitting or receiving data and as “idle” when it is in either a listening state (i.e., listening but not actually receiving data) or a sleeping state (i.e., not listening although the other sensors may be on). Sensor radios must wakeup periodically to see if other nodes or a local node want to send data. If not, then they may go back to sleep. An internal mechanism such as a scheduling algorithm is used to decide when to change from listening to sleeping and vice versa. For example, if a node remains asleep for time TS then it undergoes a transition to the idle listening state. During the listening state if data arrives from neighbor nodes or it has data to send, the local node changes to the active state. Otherwise, it returns to the sleeping state after TL. After data transmission or reception is finished, the sensor node transitions from the active to the idle state. In a conventional scheme, the nodes must wakeup at the same time to communicate with each other. If time is not synchronized, each node will wakeup at a different time. To maintain the network synchronized, nodes need to exchange messages from time to time to synchronize their clocks with each other and prevent clocks from drifting away.
Due to energy, cost and physical constraints, sensor networks present various design, implementation and deployment challenges. Energy efficient protocols at various levels (i.e. physical layer, MAC layer, logical link control layer, network layer, etc.) are extremely critical for sensor networks. Since the environment is noisy and some nodes can be mobile, the MAC protocol and other link layer protocols must be power-aware and able to minimize collisions with neighbor node's communications.